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**Stacks, Queues, and Linked Lists**

Stacks (Last in/First out List) Operations: Push, Pop, Test Empty, Test Full, Peek, Size Queue(First in/First out List) Operations: Insert, Remove, Test Empty, Test Full, Peek, Size Linked List(A list of elements connected by pointers) Insert, Delete, Find, Traverse, Size Advantages: can grow, delete/insert with assignments

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**Complexities Queue Stack: List Insert O(1) Remove O(1) Peek O(1)**

isEmpty O(1) isFull O(1) List Search O(N) Stack: Push O(1) Pop O(1) Peek O(1) isEmpty O(1) isFull O(1)

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**Stack Implementation With an array With a linked list**

Need stack array and top members. Push: (top<MAX) ? stack[++top] = data : <error>; Pop: (top>0) ? return stack[--top]:<error>; With a linked list Push: insert at front of list. Pop: remove from front of list. What are the complexities?

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**Stack: Array Implementation**

int top, arraySize; Data *array[]; void Stack(int s) { array = malloc(sizeof(Data)*s; arraySize = s; top = -1; } int push(Data *s) { if (isFull()) return false; array[++top] =s; return true; } Data *pop() { if (isEmpty()) return null; return array[top--]; } int isEmpty() { return (top < 0); } int isFull() { return top+1 == arraySize; } Data *peek() { if (isEmpty()) return null; return array[top];} Note: In C, non-zero = true, zero = false

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**Stack: Linked List Implementation**

Data *top; Stack() { top = NULL; } void push(Data* d) { d->link=top; top=d; } Data *Pop() { if (isEmpty()) return null; Data *value = top; top = value->link; return value; } int isEmpty() {return (top == NULL);} Data *peek() { return top; }

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**Queue Implementation With an array With a linked list**

Circular Queue Need queue array, size, head, and tail pointers. Insert: (size<MAX) ? queue[++tail%MAX] = data : <error>; Remove: (size > 0) ? return queue[++head%MAX] : <error>; With a linked list Insert: Insert to back of queue Remove: Remove from front of queue. What are the complexities?

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**Queue: Array Implementation**

int head, tail, entries; int size; Data *array[]; public Queue(int s) { array = malloc(sizeof(Data)*s); size = s; head =0; tail = -1; entries = 0;} public boolean insert(Data s) { if (isFull()) return 0; if (tail==size) tail = -1; array[++tail] = s; entries++; return 1; } public Data *remove() { if (isEmpty()) return NULL; Data *temp = array[head]; head = (head+1)%size; entries--; return temp; } int isEmpty() { return entries == 0; } Data *peek() { if (isEmpty() return 0; return array[head]; } }

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**Queue: Linked List Implementation**

Data *head, *tail; Queue(int s) { head =null; tail = null; } int Insert(Data *s) { if (isEmpty()) { head = tail = s } else { last->link = value; tail= s; } return true; } Data *Remove() { Data *value = head; if (isEmpty()) return NULL; head = head->link; if (head == NULL) : tail = NULL; return value; } int isEmpty() { return (head == null; } Data peek() { return head; }

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**Linked List Implementation See Text Example**

With dynamic memory The data structure uses object links. Insert/Delete: Search; assignments to change pointers With an array (Use indices instead of data links) Need to list array, size, and pointer to initial entry. Initialization: Create free entry chain. Insert: retrieve from free list if any and do normal insertion. Delete: Do normal insertion logic and then add to free list. The data structure uses primitive links rather than object links.

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**Ordered Linked List Insertion**

Item first; void insert(Item *d ) { Item *previous = null; Item *current = first; while (current!=NULL && d->key > current->key) { previous=current; current=current->next); } if (previous == NULL) first = d; else previous->next = d; d->next = current; } Note: Duplicates OK in this implementation

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Linked List Removal Item remove(Item *d) { Item *current = first; *previous = NULL; do { if (current == null) return NULL; if (!equals(current->key, d->key)) { previous = current; current = current->next; } } while (current!=null && !equals(current->key, d->key)) if (previous == NULL) first = first->next; else previous->next = current->next; return current; }

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**Doubly Linked List See Text Example**

Two links. One to next record and one to previous. Characteristics. More assignments to maintain links. Don’t need the previous temporary pointer. More memory per record. More secure. Used for operating systems.

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**Doubly Linked Insertion**

void insert( Item *d ) { Item *current = first, *previous = NULL; while (current!=NULL&&compareTo(d->key, current->key)<0) { previous = current; current=current->next); } if (current == NULL) { if (previous == NULL) first = current; else previous->next = d; } else { if (current->previous==NULL) { first=d; d->next=current; current->previous=d;} else { d->previous = current->previous; d->next = current; current->previous->next = d; current->previous = d; }

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Doubly Linked Removal int remove(Item *d) { Item *current = first; do { if (current == NULL) return NULL; if (current->key != d->key) {current = current->next; } } while (!equals(current->key, d->key)); if (current->previous == NULL) first = current->next; else current->previous->next = current->next; if (current->next != NULL) current->next->previous = current->previous; return true; }

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**Stack Examples Matching pairs of delimiters**

Evaluating infix expressions Two stacks First convert to postfix and then evaluate Expression examples {…{….[..{.<.{..{…}…}…>..].}..} 500/(1+2*(3+4*5/2))*(3*2+1)

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